I add 7/8 in to the size I want the triangle to be. - Such as : for a triangle from a 3 in sq. cut the square 3 7/8 in and you should have the correct size. :!:
I am presently doing 8.5" squares and made my squares 7.5" to cut in half diagonally. I don't have as much waste that way and it came out for me. Am I completely wrong in this?